Mechanism of the parasitic remanence of aluminous goethite [α‐(Fe, Al)OOH]

The Néel temperature ( T N ) of cation‐substituted goethites is controlled by the degree of isomorphous substitution and crystal defects. In particular, the exact relationship between the T N of the antiferromagnetism (AFM) and the unblocking temperature ( T B ) of the parasitic remanence is undetermined. We found that T B is systematically higher than T N for a set of well‐characterized aluminous goethite samples. In addition, the difference between T B and T N increases from ∼8–9 K for a pure goethite (that contains vacancies) to >20 K for Al‐substituted goethites. This indicates that T B and T N change independently with the diamagnetic substitutions, suggesting a fundamental difference of magnetization process between parasitic remanence and AFM. A zonal distribution of the diamagnetic substitutions has been incorporated to account for the observed magnetic properties of the AFM and the parasitic remanence. Nonetheless, both parasitic remanence and AFM could exist along the goethite c axis.